General and experimental psychology

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1. Crossmodal interval interaction in Ternus apparent motion


Ternus apparent motion (or Ternus illusion) was first discovered by Josef Ternus in 1926. Using two frames of multiple dots, when overlaid, share one or several common dots at the center. Depending on the spatial configuration and inter-stimulus-interval (ISI), two distinct motion percepts are usually observed: 'element motion' (outer dots flip) and 'group motion' (all dots shift together). Short ISIs usually give rise to the percept of 'element motion', while long ISIs give rise to the perception 'group motion'. A great interactive demo can be found in Michael Bach's visual illusion website: Pikler-Ternus Display demonstration.

Based on classical Ternus apparent motion paradigm, recently we (Shi, Chen & Müller, 2010) examined multisensory temporal interactions on the visual apparent motion, and found that spatial uninformative auditory stimuli can modulate the visual motion percepts. In particular, auditory interval defined by short beeps influenced the perceived visual interval (defined by visual Ternus frame). As a result, two motion percepts were changed accordingly. (see detail in our paper: DOI: 10.1007/s00221-010-2286-3)

In the following demo, two different sound intervals are presented together with same visual Ternus display. Try it yourself to see how auditory sound influence visual apparent motion using the two buttons below.



  • If you can not see above demo in your browser, please click this demo link.
  • due to auditory lagging with your browser, be aware the effect may be small.
  • please fixation  on the center of fixation cross, you may get different percepts.


2. Anisotropic flash-lag, movement-mislocalization effects

Magnitudes of the flash-lag effect can be influenced by various factors. One of such factor is fovea-related motion (foveopetal and foveofugal motion). Motion from periphery to central vision (foveopetal motion) often causes a greater flash-lag effect than the motion in the opposite direction (foveofugal motion). In one of our study (Shi & Nijhawan, 2008), we identified that two factors contributed to such asymmetric flash-lag effect, namely, mislocalization of the flash caused by the motion and the mislocalization of the moving object per se.

The following link will pop-up a demo show the asymmetric flash-lag effects between two different direction of motions. If you are interested in this, please check the following references.

Demo of anisotropic flash-lag effects.

3. Motion extrapolation in the fovea

Flash-lag effect can be observed during the motion or at the motion initiation. However, the flash-lag effect is often absent at motion termination (Demo download).  A recent 'correction-for-extrapolation' hypothesis (Nijhawan, 2008) suggests that the absence of forward shifts arises from biased competition process due to strong transient stop signals. To demonstrate the motion extrapolation in the central fovea, one has to physically reduce (or remove) the positional signal. In our recent serial studies, we used a dim and a blue moving objects and successfully observed the motion extrapolation at motion terminated condition.

It is well known that in the central fovea there is a 0.3 mm diameter rod-free area, where the low intensity objects fail to yield a visual percept (Hecht, 2002). Thus a continuous dim moving object moves across rod-free fovea, one would perceive a discontinuous movement.  If the motion percept follows faithful to the retinotopic map (no motion extrapolation for the neural delay),  one should see a discontinuous movement is symmetrically located near fovea (i.e. the dim moving object vanishes at the boundary of rod-free fovea, and reappears at the boundary of fovea, as illustrated in the following figure).


Without motion extrapolation

However, the motion extrapolation hypothesis would predict that the dim object moves into the fovea (extrapolation) and reappears further away from the fovea (a Fröhlich effect). The following figure shows how you would observe with the dim moving object.


With motion extrapolation

To see such asymmetric extrapolation phenomenon, please download the following flash demo, and then open with full screen.

Demo of motion extrapolation in the fovea [download].


  • This scotopic illusion can only be seen in a dark room with a low luminance. So please reduce the brightness and contrast of your monitor, such that you barely see the moving object. In addition, you need several minutes of the dark adaptation for a better illusion.

  • Alternatively, you can use a Tokyo blue filter, through which you should be able to see the illusion in a normal light condition.


  • Shi, Z., & Nijhawan, R. (2008). Behavioral significance of motion direction causes anisotropic flash-lag and flash-mislocalization effects. Journal of Vision, 8(7):24, 1-14, [DOI: 10.1167/8.7.24].
  • Mateeff S., Hohnsbein J.(1988). Perceptual latencies are shorter for motion towards the fovea than for motion away. Vision Research, 28, 711–719.
  • Shi, Z., Chen, L., Müller, H. J. (2010) Auditory temporal modulation of the visual Ternus effect: the influence of time interval, Experimental Brain Research, 203(4), 723-735.
  • Shi Z, Nijhawan R, 2010, "Motion extrapolation in the central fovea" Perception 39 ECVP Abstract Supplement, page 138
  • Shi, Z., & Nijhawan, R. (2012). Motion extrapolation in the central fovea. PloS one, 7(3), e33651. doi:10.1371/journal.pone.0033651